1. Field of the Invention
The present invention relates to a process for fabricating masks for use in X-ray lithography, and particularly, it relates to a technique for forming, with higher base selectivity, mask patterns by dry etching an X-ray absorbing metal layer on an X-ray transmitting film.
2. Prior Art
Semiconductor integrated circuits are becoming more highly integrated every year, and the degree of integration is stably increasing at a constant rate, i.e., it is doubled in three years. Accordingly, severer requirements are demanded for fine processing; for instance, a fine processing technology capable of realizing a pattern with a minimum line width of 0.2 .mu.m or less is believed to be indispensable for the advanced device, i.e., a 64-M DRAM.
It can be safely said that the technological progress in lithography has played the main role in realizing such a high degree of integration. Near ultraviolet light, far ultraviolet light, and the like are used for exposure in the present day lithography. However, the resolution achievable with a beam of light in the aforementioned wavelength region is limited. Moreover, the use of a light with shorter wavelength lowers the depth of focus. This is another problem to be solved.
In the light of the aforementioned circumstances, X-ray lithography is extensively studied as an advanced technology in the field of lithography. Since an X ray is an electromagnetic wave having an ultra-short wavelength and a high power of penetration, fine processing call be effected, without being scattered by the substance and almost free from the influence of diffraction. Accordingly, a sufficiently long depth of focus can be attained without being influenced by the particles that are present on the wafer.
On the other hand, however, several problems must be overcome to put X-ray lithography into practice. One of the problems is the ultra-fine processing of the mask pattern. Since a reduced projection optical system is unfeasible in the case using X ray, the pattern transferred to the wafer must be the same as the mask pattern.
In general, a mask for use in X-ray lithography comprises an X-ray absorbing metal layer, an X-ray transmitting film to support the metal layer, and a frame to support the X-ray transmitting layer by the outer periphery.
The aforementioned X-ray absorbing metal layer is made of a heavy metal having a large atomic number, such as gold (Au), tungsten (W), and tantalum (Ta). Since the heavy metal layer is mainly processed these days by a dry process, representatively RIE (reactive ion etching), tungsten, tantalum, etc., are generally preferred to gold, because gold exhibits poor reactivity with other elements.
The X-ray transmitting film is made of a material having excellent transmittance of soft X rays. More specifically, compounds of relatively light elements such as silicon nitride (SiN), silicon carbide (SiC), and boron nitride (BN), are used.
The frame must be sufficiently rigid to support the X-ray transmitting film, and is made of a material which can be etched partially; usually, it is made of, e.g., Si.
When the X-ray absorbing metal layer is patterned by dry etching, a highly selective X-ray transmitting film must be provided as the underlying film. For instance, dry etching of a tungsten layer, which is a most commonly used X-ray absorbing film, must be performed by using a reaction product capable of yielding a sufficiently high vapor pressure. In this context, a fluorine-based compound is used as the etching gas to utilize fluorine radical (F*) as the principal etching species. When an SiN film is used as the underlining X-ray transmitting film, however, a sufficiently high base selectivity cannot be assured in a system using F* as the principal etching species unless some measure is taken to protect the surface. Still, none of the gases generally used in the field of dry etching provides a solution to this problem.
For instance, one may think of protecting the surface and the side walls by depositing carbon based polymers, thereby securing the high selectivity of the base layer. However, a process which generates particles at a large amount is not desired in the case of such a fine processing in which a minimum line width of 0.2 .mu.m or less is implemented. As a matter of course, the use of chlorofluorocarbon (CFC) gas, i.e., the so-called Freon gas, must be left out of consideration from the viewpoint of the future regulations cast on Fleon gases.
One may otherwise propose the use of other halogen radicals such as Cl* and Br* as the principal etching species. However, the vapor pressure of the reaction products resulting from these halogen radicals, i.e., tungsten chloride (WCl.sub.x) and tungsten bromide (WBr.sub.x), is far lower than that of tungsten fluoride (WF.sub.x). Accordingly, etching cannot be performed at a practically useful rate using halogen radicals other than of fluorine, unless a high ion energy is input to accelerate the dissociation into radicals. Accordingly, a high base selectivity cannot be achieved in the case halogen radicals other than those of fluorine are used.